Vestibuloocular reflex of the adult flatfish. III. A species-specific reciprocal pattern of excitation and inhibition

In juvenile flatfish the vestibuloocular reflex (VOR) circuitry that underl ies compensatory eye movements adapts to a 90 degrees relative displacement of vestibular and oculomotor reference frames during metamorphosis. VOR pa thways are rearranged to allow horizontal canal-activated second-order vest ibular neurons in adult flatfish to control extraocular motoneurons innerva ting vertical eye muscles. This study describes the anatomy and physiology of identified flatfish-specific excitatory and inhibitory vestibular pathwa ys. In antidromically identified oculomotor and trochlear motoneurons, exci tatory postsynaptic potentials (EPSPs) were elicited after electrical stimu lation of the horizontal canal nerve expected to provide excitatory input. Electrotonic depolarizations (0.8-0.9 ms) preceded small amplitude (<0.5 mV ) chemical EPSPs at 1.2-1.6 ins with much larger EPSPs (>1 mV) recorded aro und 2.5 ins. Stimulation of the opposite horizontal canal nerve produced in hibitory postsynaptic potentials (IPSPs) at a disynaptic latency of 1.6-1.8 ms that were depolarizing at membrane resting potentials around -60 mV. In jection of chloride ions increased IPSP amplitude, and current-clamp analys is showed the IPSP equilibrium potential to be near the membrane resting po tential. Repeated electrical stimulation of either the excitatory or inhibi tory horizontal canal vestibular nerve greatly increased the amplitude of t he respective synaptic responses. These observations suggest that the large terminal arborizations of each VOR neuron imposes an electrotonic load req uiring multiple action potentials to maximize synaptic efficacy. GABA antib odies labeled axons in the medial longitudinal fasciculus (MLF) some of whi ch were hypothesized to originate from horizontal canal-activated inhibitor y vestibular neurons. GABAergic terminal arborizations were distributed lar gely on the somata and proximal dendrites of oculomotor and trochlear moton eurons. These findings suggest that the species-specific horizontal canal i nhibitory pathway exhibits similar electrophysiological and synaptic transm itter profiles as the anterior and posterior canal inhibitory projections t o oculomotor and trochlear motoneurons. Electron microscopy showed axosomat ic and axodendritic synaptic endings containing spheroidal synaptic vesicle s to establish chemical excitatory synaptic contacts characterized by asymm etrical pre/postsynaptic membrane specializations as well as gap junctional contacts consistent with electrotonic coupling. Another type of axosomatic synaptic ending contained pleiomorphic synaptic vesicles forming chemical, presumed inhibitory, synaptic contacts on motoneurons that never included gap junctions. Altogether these data provide electrophysiological, immunohi stochemical, and ultrastructural evidence for reciprocal excitatory/inhibit ory organization of the novel vestibulooculomotor projections in adult flat fish. The appearance of unique second-order vestibular neurons linking the horizontal canal to vertical oculomotor neurons suggests that reciprocal ex citation and inhibition are a fundamental, developmentally linked trait of compensatory eye movement circuits in vertebrates.